Rapid Recovery of Metals in CCA-treated Wood

ABSTRACT

The recovery of heavy metals from chromated copper arsenate (CCA)-treated southern pine wood particles was investigated using binary acid solutions consisting of two of acetic, oxalic, and phosphoric acids in a microwave reactor. Formation of an insoluble copper oxalate complex in the binary solution containing oxalic acid was the major factor for low copper removal. Furthermore, the possible complexation of acetic/oxalic acid in the organic phase, the decomposition of oxalic acid in acetic acid at high temperatures, and the promotion of the formation and precipitation of the copper oxalate by phosphoric acid may induce an antagonistic effect which adversely influenced the effectiveness of the copper extraction. It was found that the addition of acetic acid into phosphoric acid enhanced the chromium recovery rate of the mixed acid solution. This synergistic effect of mixed acetic acid and phosphoric acids is considered one of the most interesting and significant discoveries in the study. The minimal reaction conditions for extracting the maximum percentage of metals was 2.75% phosphoric acid, 0.5% acetic acid, and 130°C. The total recovery rate approached 100% for arsenic, 96.7% for chromium, and 98.6% for copper in a one step process.

INTRODUCTION

Preservative-treated wood products are well known to significantly prolong service life, and thereby extend the forest resource and enhance its sustainability. Inevitably, however, the treated products will become unserviceable either due to mechanical damage or failure, biological deterioration, or obsolescence. It is estimated that approximately 12 million m3 per year of spent CCA treated wood will be removed from service in the United States and Canada in the next 20 years (Kazi and Cooper 2006). Disposal of this material has become a major concern because of its residual toxic chromated copper arsenate (CCA) content, in particular the arsenic and chromium. Previous studies have shown that CCA compounds can be gradually leached into the environment (Townsend et al. 2005; Moghaddam and Mulligan 2008). Conventional waste disposal options for spent preserved wood, such as land-filling, are becoming more costly or even impractical because of increasingly strict regulatory requirements and liability concerns. The burning of treated wood may be extremely dangerous to the environment and human health, particularly if the wood has been treated with CCA. There is an urgent requirement for the development of techniques to effectively recycle decommissioned CCA-treated wood.

Fixation is a chemical process in which the preservative chemically bonds to the wood. It is well recognized that exposure of CCA-treated wood to an acid solution can re-oxidize the chromium thereby converting the CCA elements into their water-soluble form. Thus, acid extraction using different acids and wide ranges of reaction conditions has been extensively studied for removal of CCA from out-of service CCA-treated wood (Shiau et al. 2000; Clausen 2004; Humer et al. 2004; Kazi and Cooper 2006; Gezer et al. 2006; Kakitani et al. 2007; Janin et al. 2009). These studies have shown that the recovery of CCA elements from CCA-treated wood can be obtained with many organic acids, such as oxalic, acetic, citric, and formic acids; however, the acid extraction process is slow. Therefore, cost-effective acid extraction methods are lacking.

Our recent studies on removal of CCA elements from spent CCA-treated wood have focused on the application of the microwave energy to facilitate acid extraction. A preliminary study (Yu et al. 2009) has shown: (1) microwave-assisted acid extractions with oxalic, acetic, and phosphoric acids have reduced the reaction times from hours to minutes compared to the conventional methods, (2) oxalic acid effectively removed arsenic and chromium but not copper, (3) acetic acid extraction was highly effective for the removal of arsenic and copper but not for chromium, and (4) extraction using phosphoric acid was less effective as compared to both oxalic and acetic acids. These results indicated that none of the individual acids were able to effectively remove all three CCA elements simultaneously, but showed a potential complementary effect for extraction. For instance, oxalic acid removed chromium very effectively but not copper, and acetic acid effectively extracted copper but not chromium. The results strongly suggested the opportunity for a two-acid process by either a synergistic extraction effect of the combined acids or a two-step process of consecutive acid extraction. In this study, two acids were mixed, and the extraction potential was evaluated. The acids studied were oxalic, acetic, and phosphoric. The objective of this study was to develop a cost-effective microwave-based dual acids extraction system to maximize removal of CCA elements from spent-CCA-treated wood. This was addressed by optimizing of the acid combinations and concentration, extraction times, and reaction temperature to minimize any environment impact.